The present invention relates to a process and apparatus for producing wood pulp using a wood grinder and wherein water is fed to the grinding zone. The grinding is effected in a pressurized grinding zone and the water temperature is kept over 100.degree. C. This invention is an improvement upon the disclosure of U.S. application Ser. No. 21,070 filed Mar. 16, 1979 by Helmut Thumm, entitled "Process and Apparatus for Producing Wood Pulp", commonly owned by the Assignee hereof.
To grind wood under a positive pressure above atmosphere, the housing in which grinding is performed is sealed from the atmosphere. Because the housing is pressurized, it is possible to grind with water at higher temperatures. This produces better and more uniform quality wood pulp, in which the proportion of long fibers and the tear length of the wood pulp are increased. For further details concerning this process and concerning apparatus suitable for performing the process, see the aforesaid application.
After the wood has been ground into pulp, it is necessary to bring the wood pulp, which has a pulp consistency range of about 1 to 2.5 percent of absolute dryness, back to atmospheric pressure.
Upon the discharge of the wood pulp from the grinder housing, a very large amount of heat also leaves the parts of the apparatus which are under pressure. This heat can be recovered only by means of relatively large and complicated heat-recovery systems. Furthermore, steam washing and chemical steam neutralization are also necessary. Due to the low temperature of the vapors recovered, the waste heat can be used in this connection only for the pre-heating of the water to be used in the grinding zone up to 100.degree. C. But water at a higher temperature is required for grinding effectively. In order to maintain the heat equilibrium required for the grinding process, therefore, two to three tons of steam per ton of pulp per hour must be supplied to the grinding zone.